The Zebrafish Information Network (ZFIN) is an online biological database of information about the zebrafish (Danio rerio). The zebrafish is a widely used model organism for genetic, genomic, and developmental studies, and ZFIN provides an integrated interface for querying and displaying the large volume of data generated by this research. To facilitate use of the zebrafish as a model of human biology, ZFIN links these data to corresponding information about other model organisms (e.g., mouse) and to human disease databases. Abundant links to external sequence databases (e.g., GenBank) and to genome browsers are included. Gene product, gene expression, and phenotype data are annotated with terms from biomedical ontologies. ZFIN is based at the University of Oregon in the United States, with funding provided by the National Institutes of Health (NIH).